Starting today, it was impossible to get the profile up, I was presented with a blank page.
for the moment, my workaround was to install thunderbird on my PC, click on troubleshoot, then about:profiles, and finally create a new profile by pointing to my exiting one, copied from the portable installation, and making that the default. (the portable application has to be closed at that point, or it won't allow you access).
I couldn't direct to the portable profile from the portable app, it defaults to local appdata and won't redirect to the portable.
Sorry for the garbled explanation. I'm hoping it will save someone a couple of hours frustration.